1. Field of the Invention
The present invention relates to a toner for use in a recording method using an electrophotographic method, an electrostatic recording method and a toner jet recording method.
2. Description of the Related Art
Previously, a large number of electrophotographic methods are known. A copied article is typically obtained by using a photoconductive material, forming an electrical latent image on an image bearing member (photosensitive body) by a variety of means, then obtaining a visible image by developing the latent image with a toner, transferring the toner image on a transfer material such as paper, as necessary, and then fixing the toner image on the transfer material by heat or pressure.
In recent years, a demand for inexpensive and small-size copiers and printers has grown following the rising popularity of such devices using the electrophotographic method, including household use thereof. In particular, in terms of cost efficiency and environment, the attention has been focused on the development of energy-efficient devices.
From the standpoint of energy efficiency, electrophotographic toners used in copiers and printers are required to have a low fixation temperature which results in low power consumption. To meet such a requirement, attempts have been made to design a toner with the lowered glass transition temperatures (Tg) of the binder resin and wax used therein or with the lowered melting temperature of the wax. However, such designs resulted in degraded stability in storage of the toner. Furthermore, under a high-temperature environment, the low-molecular weight components contained in the binder resin or the wax easily seeps out to the toner surface, thereby easily causing the aggregation of toner particles or filming.
A toner with a core-shell structure in which the surface of a resin serving as a core is covered by a shell resin has been suggested to resolve this problem.
Japanese Patent Application Laid-open No. 2009-163026 suggests a toner using materials with high affinity as the resins constituting the core and the shell, those materials having close solubility parameter values (SP values). According to this document, since the core is covered by the shell that has adhered thereto, the wax can be prevented from exude, and heat resistance in storage and stability of the fixed image are improved. However, when the inventors have checked this technique, it was found that under severe conditions such as repeated variations in temperature and moisture environment, exude of the wax still can occur and the exude inhibition effect is insufficient.
Japanese Patent Application Laid-open No. 2010-168522 describes an example in which a compound having an organopolysiloxane structure is used as a toner shell resin. Organopolysiloxane compounds are known as materials typically having a low solubility parameter value (SP value). The inventors have assumed that the presence of such a material with a low SP value on the toner surface will apparently be capable of preventing the wax from exude under the above-mentioned severe conditions. However, with such a technique, the difference between the SP value of the shell resin and the SP value of the core binder resin is increased. As a result, the adhesiveness of the core and the shell is low and a sufficient core-shell structure is not created which is apparently why the core was found to seep out when the technique was verified.
Japanese Patent Application Laid-open No. 2006-91283 suggests a toner of a core-shell structure comprising a binder resin and an organopolysiloxane compound in a shell resin. According to this document, the toner obtained excels in ability to separate from the thermal fixation roll, and an image with long-term stability can be obtained. When the inventors have estimated the toner obtained in this document, the exude of wax was actually found to be inhibited. However, at the same time, low-temperature fixation was found to be difficult. The reason therefor is apparently that since the organopolysiloxane compound is contained in the core, the exude of the wax is also inhibited during the fixation and a cold offset easily occurs. Yet another reason is apparently that the shell resin is used in a large amount of about 20 parts by weight to 60 parts by weight per 100 parts by weight of the core, and the shell phase is thick. Therefore, the core is unlikely to obtain the sufficient amount of heat from the thermal roller during the fixation.